This invention relates to a method for preventing soldering of high purity aluminum to steel, and, more particularly, to the mass production of squirrel-cage rotors for electric motors.
It is well known to produce squirrel-cage rotors by stamping a plurality of generally circular, high magnetically permeable laminations from thin steel sheet stock. The laminations each include a central bore and a plurality of identical generally radial notches spaced around the circumference at equal angular intervals about the outer margin of the lamination. The laminations are then stacked and compressed within a die casting mold to form a core having a longitudinal central bore there through and slots spaced around the circumference which extend longitudinally through the core at the outer margins thereof. The laminations are skewed such that the slots are wrapped slightly around the longitudinal axis of the core in a somewhat helical fashion. Molten metal is then injected into the slots formed by the laminations to produce spaced bars along the outer margins of the core, as well as end rings which hold the laminations in place.
It is also known that in order to produce the very best motor performance possible, the electric conductivity of the bars should be as high as possible. It has been generally accepted that the bars should be formed from the highest purity aluminum, and thus the highest conductivity aluminum, which is available. The aluminum which has been generally utilized by motor manufacturers has a very low iron content of about 0.1% to 0.2% by volume.
It is known in the art that molten aluminum is very aggressive toward unprotected steels, with the result that molten aluminum often solders to unprotected steels. It was also common in past motor manufacturing procedures to heat treat laminations after punching to mitigate aluminum soldering in rotor casting. That is, stator and rotor laminations often still are heat treated, or acid etched to form an oxide layer on the bare metal to help prevent the aluminum from soldering to the steel. When oxidation steps are provided in a motor construction, they add cost to the product and the degree of oxidation is hard to control. Consequently, even where oxidation steps are included in the motor manufacturing process, it still is possible to have production problems with rotors using conventional construction techniques.
Sodium borate has been used in the past in an attempt to form an electrically resistive barrier on both the lamination interfaces and the rotor bar slots. The sodium borate alone, however, has not been successful for this application.